Wave modulation and application thereof



June 16, 1931.

E.. PETERSONv WVE'MO-DULATION AND APPLICATION 'Il-IEREOF Filed Oct. 30I 1926 Patented June 16, 1931 UNITEDl STATES PATENT OFFICE EUGENE PETERSON, OF N EW' YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK WAVE MOIOULATION AND APPLICATION THEREOF Application led October 30, 1926. Serial No. 145,113.

This invention relates to signaling, and especially to modulation and demodulation.

According to this invention undesired magnetic coupling between the input circuits of a. magnetic modulator is avoided by having the carrier frequency flux and the signal frequency flux at rlght angles Where they are superposed in the magnetic structure of the modulator.

According to another aspect the functions of a demodulator and telephone receiver are combined in a singlemagnetic device by effecting demodulation in the magnetic core of the device to produce a. flux of signal frequency, which flux cooperates with a steady flux to actuate the receiver diaphragm.

Another feature is the transmission of a signaling side frequency of a carrier wave impressed on a modulator, with suppression of the first harmonic of the carrier wave, and transmission of a signal other than that employing the side frequency, by transmission of the lirst harmonic of the impressed carrier wave.

Other features of the invention are de-V scribed hereinafter.

Figs. 1 and 2 show curves for facilitating explanation of the invention; Fig. 3 is a diagram of a two-way, multiplex telephone system involving features of the invention which are referred to above; and Fig. 4 is a diagrammatic side elevation of a receiver demodulator structure included in the system of Fig. 3. Y

The term modulation as used herein, relates to distortion of waves,'and especially to phenomena which occur when two electric waves, for example, a so-called carrier wave and a so-called modulating wave, are jointly impressed upon a distorting instrumentality. l

Itis well known that modulation of this kind yields side frequencies made up of various combinations involving the frequencies of the impressed waves. When one of the waves, either the modulating or carrier wave consists of a bandof frequencies the side frequencies likewise assume the form of bands.

The number, amplitude, and frequency characteristics of these side bands are functions of the kind and extent of the distortion as measured, for example, by the equation of the characteristic curvewhich expresses the relation between the amplitude of the input and output quantities. i

In present systems, use is made of the square charactistic of this curve, that is, the characteristic which makes the equation of the curve approach the form g/=c m2, in which and' g/ are respectively the amplitudes of the input and output quantities. Other terms having other powers of :v may, to some extent, lbe present. Those in other even powers of will contribute to the effect secured by the'square lterm as will be explained later. On account of this square characteristic, modulation yields side band components having frequencies equal respectively to the sum and difference of the frequencies of the modulating and carrier waves. These side bands with or without some of the original carrier constitute a modulated carrier wave from which the modulating component may be reproduced at a receiving station by a similar distortion. Y

The terms besides the square term, if present, in addition to contributing to the effect of the square term, as above, produce other side bands in which harmonics of the carrier or modulating frequencies, or both, instead of the fundamental frequencies, occur. These various combination vfrequency components or side bands are distinguished from each other by what may be called their order, or the number'of times the carrier and modulating frequencies, taken together, occur. iining the' orderv of modulation which produces these components. For example, second order modulation produces second order side bands or side bands of the kind usually employed, in which the carrier and modulating frequencies each occurs once, third order of modulation results in third order side bands or side bands in which the first even harmonic of either the carrier or modulating fre uency occurs, etc.

oduleion of order higher than the second may be conveniently designated higher order modulation, or high order modulation;

This distinction is made use of in deand side bands' of higher order than'the sec- 0nd may be designated higher order or high order side bands.

Pronounced higher order side bands can be produced by modulating means which produce, and operate over, a characteristic 'curve -which has one or more ronounced bends. Where it is desirable, as or example for reasons pointed out hereinafter, that odd order side bands be obtained to the exclusion of the even order side bands, the curve should have two symmetrical bends in relatively opposite directions, that is the curve should have opposite symmetry about each of the axes of' coordinates through a point on the curve and the operation should occur equally over the curve in both directions from this point about which the curve is symmetrical. If the operation does not occur about the point of symmetry or if the curve lacks symmetry of this kind, even order side bands will be present. In general, for either case the degree of 'acuteness of the bends indicates proportionalllyI the amplitudes of the side bands.

odulation producing pronounced odd ord'er side bands to the substantial exclusion another winding on the core, can deliver mod- 40 ulated current. rlhe modulator may be regarded as accomplishing its function by varying the flux density of the core, and therefore the impedance or the mutual. inductance of the carrier current and side band current windings, as a non-linear function of the modulating current. Such a magnetic core modulator when operatin with .sym-

lmetry about a point on its c aracteristic curve suppresses, for example, the double carrier frequency as well as the even order side bands, and since it is easy to separate the vfundamental carrier frequency. from the third order side bands by a band filter, the

use vof this type of modulator facilitatestransmission of a third order side bandl with suppression of the fundamental and double carrier fre uencies, a type of transmission utilized in t e system shown in the drawing and described hereinafter.

`If currents of dierent frequencies p and g are fed into a distorting (modulating) device, there results a complex current in which, as indicated by a mathematical analysis given presently, the frequency of each current component can be represented by the general expression m pin g, in which m and n may have any or all integral values, or zero values, and in which the symbol iindicates that the sum, the difference, or both the sum and difference, of the two quantities may be present.

When either of the coefficients m or n has zero value,-the frequency defined 1by this expression is a direct current if the other coeilicient is zero. If the other coeilicient is not zero, the current component has a frequency p or g, as the case may be, or a harmonic or harmonics thereof. When neither of the coefficients has zero value there are obtained combination-frequency components of current, that is, side frequencies. The

-order of modulation is conveniently given as thesum of m and n. When both m and n are unity we have the familiar case of second order modulation in .which the side frequencies are pig. Third order modulation may be correspondingly represented by 2p i q or piQg. There are accordingly four possible third order side frequencies. If a is a number expressing the order of modulation (that is, a number which is the sum of two numbers) it is evident that there are r-1 combinations of integral numbers (other than zero) which add up to equal r, so that, considering both the sum and difference frequencies, there may be a maximum of- 2 (r-l) differentl side frequencies for each of the respective orders of modulation.

l In what follows, it will be assumed that g represents a band of frequencies as, for example, a voice -current (the so-called modulating current) and that p represents a fixed carrier frequency having a value greater than the eatest value of g. The expression side ban will accordingly be used rather than side frequency. Also, only the side bands in which g occurs but once, that is, those having frequencies p i Q, 2171-51, 3p g, etc., will be considered, since only by the transmission of side bands of thisV type can the modulated component be reproduced at the receiver by modulating such side bands with a wave having the frequency of the unmodulated carrier or a harmonic thereof.l

In order to show how various multiple` order side bands occur and to determine the extent to which they occur, an extension of the method of analysis used in United States patent to` Carson, No. 1,449,382, issued March 27, 1923, is given below.

According to that method a statement of the k current (or potential) resulting from modulation is lobtained by substituting in the general equation of the type yf=a+ bz2 am values of the simultaneouslyimpressed currents (or potentials). Suppose that the input currents are Pocos p1 t and Q cos g1 tv, in which p1v and glequal respectively 2IIp and 211g, so that a: equals P cos p1 t-l-Q cos q1 t.

(No material changewould result if an initial phase angle between the two impressed waves were assumed.)

This value of w should be substituted in the general equation justc given. The first term am yields merely amplified waves lof the impressed frequencies p and g. The term 5:02 yields waves of frequencies 2p, 2g, and pig, as is Well known. The second order side bands p 1*: g result from the trigonometric vexpansion of the product b PQ cos p12? cos Q11?.

The harmonic frequencies 2p and 2g result from trigonometric expansions of, respeccombination frequency waves it will be found that alternate terms beginning with the second in the expansion of even power terms of the general equation are of this type and are further characterized in that the exponents of cos p16 and cos g1# are each odd. These terms in their regular order, up to and-including the expansion of the sixth power term of the general equation, are as follows, only those coefficients which are necessary in this discussion being retained:

P@ cos 11115 cos glt P3@ cos3 p11*, cos glt P@3 cos p12? cos3 g1# P5@ cos5'p1t cos glt P3@3 cos8 plt cos3 gli P@5 cos plt cos5 gli The frequency determining quantities in these terms are each in the form cos pit cos gli? or this quantity times one or more cosine squared quantities.l Since cos2a= 1/2 -ll/cos 2a `the development of these quantities each contains one term of the form of 1/2 cos plt cos g] t. This demonstrates that the terms of (l) each yields apair of second order side bands. These side bands for the several terms are superposed to comprise resultant side bands.

The amplitude of each of the resultant upper and lower side bands may accordingly be expressed as a series the terms of which are proportional to PQ, PSQ, PEQ, PQ?, PQS;

P3Q2-all being products of even order.

These terms will be multiplied by the coefficients 6,03, f, etc., of the 'general equation. In general these cociicients decrease in magnitude as the power of the terms of the general equation increases, that is, as the order of the above products increases. In fact the characteristic curve may be caused to be substantially square so that substantially only the term PQ, which is linear in Q, is present. For other cases the other terms will be present in relatively small amounts and will introduce some distortion on account of non-linearity of certain of the coefficients in Q. However, a number of the' terms as PQ, P3Q, PSQ, etc., hence their sum, will be linear in Q. The magnitude of the distorting terms, that is, PQ3, PQ5, PSQ", etc., may be minimized by making P large as compared with Q, so that substantial linearity may be obtained, as is necessary for distortionless transmission. This is true even though the characteristic curve departs widely from its square conguration.

It may be shown, ina manner similar to the above, that other side bands of a different even order may be obtained from other terms than those indicated in (l) in the expansion of the even power terms of the general equation. However,'as will be more evident later,

only those of the form ml pig will have coefficients which are linear in `Q and, therefore,

useful.

From the odd power terms of the general equation odd order side bands may be similarly obtained. This will be demonstrated for the case of third order side bands.

From the expansion of these odd power terms (except the linear term) it will be found that alternate terms beginning with the second haveeven powers of cos p11? and odd powers of cos git. These terms in their regular order, up to and including those for the seventh power term of the general equation, are as follows, retaining only the necessary coefficients.

P2@ cos2 plt cos glt P4@ cos4 pli cos glt P2@a cos2 plz? cos3 glt P6@ cos plt cos glt P4@3 cos* 10112 cos3 gli P2@5 cos2 plt cos5 gli The frequency determining quantities in these terms are each of the form cos2 p1# cos gli or this quantity times one or more cosine squared quantities.

Since cos2a=12+12 cos 2a, the development of the portion cos2 pli cos Q12? contains the term 1/2 cos 2 plt cos glt. The product of this term with the development ofthe re.

maining cosine squared quantities results in a term made up of the product of this term and l. Accordingly, the development of each of the terms in 'contains a term of the form cos 2 plt cos gli This form is similar to the form cos pli cos gli? and in an analogous manner yields upper and lower side bands of 2p. This demonstrates that the terms in (2) each denotes a pair of third order side bands of the type 2in1-( y.

The amplitudes yof each of the resultant upper and lower side bands may accordingly be expressed as a series, the terms of which are proportional to PZQ, PQ, PQ, PZQB, PQs, P2Qj The sum of these terms is linear under the conditions discussed above for second 'order modulation. That is, when there is only the third power term in the general equation, only the first term of the series occurs. and the side bands are strictly linear in Q, and when there are additional terms, there is a slight distortion which can be reatly minimized by properly proportionmg the values of P and Q.

It may be shown in a similar manner that third order side bands of the type ptQg may be obtained from other terms than those indicated under (2), of the development of the odd power terms of the general equation. The quantities expressing the amplitude of these side bands are made up of terms none of which is linear in Q, so that the sum is not linear and cannot be made linear in Q. This means that third order modulation of this type cannot yield a faithful reproduction of the signal. The same thing is true of other higher orders of modulation, even or odd, in which n is greater than one. These side bands may, however, be used in signaling where accurate reproduction of the modulating wave is not essential.

Side bands of a higher odd order than third of the type m pig can also be obtained from the odd power terms of the general equation. These side bands, as well as the higherk even order side bands of the same type, can be made substantially linear in Q.

By the method employed above in considering high order modulation, it can be shown that high order demodulationof a modulated wave can yield the original modulating wave. For example, if a wave of frequency p1 and a third order side band or modulated wave having a frequency 2p1lg1, where p1 is the original carrier frequency and g1 is the original modulating frequency, be Jointly impressed on a wave distorting means such as, for instance, the third order magnetic modulator referred to above, there willNY result a side band of the wave p1, which will have a frequency (22h-tgl) 2121, and which will be equivalent to the original modulating wave of the frequency g1. Although telephone systems now in use depend upon second order modulation and demodulation, experience has shown that certain higher orders, especially the third, are suitable for the production of side bands and for the reproduction of speech. As illustrating the practicability of using thehigher orders of modulation it was recently found that in an actual carrier current telephone system arranged for optimum second order` modulation conditions and for transmission of the unmodulated carrier component, the amplitude of the third -order side bands could, by adjustments /not affecting the conditions ofysecond order modulation, be made of the same order of magnitude as that of the second.

It is apparent that the term carrier frequency must be re-defined for use in -describing systems of modulation of higher order than the second. For example, a third order side band having frequencies 2pig, can be demodulated to reproduce a signal by either` second or third order demodulation, depending upon whether a current of. frequency 2p or p is available. Accordingly, depending on the order of demodulation, either of these two frequencies may play the role played by the carrier frequency in a system using second order modulation (and accordingly second order demodulation). In

-this specification the terms carrier current and carrier wave will be applied to any current or wave that may be combined in a distorting device with the transmitted side band or side bands to produce a signal, and the frequency of such current or wave will ac'- cordingly be designated a carrier frequency. A modulated wave of a high order may accordingly have as carrier frequencies both the frequencies of the impressed high frequency wave and certain of its harmonics. The impressed high frequency wave will be designated as the impressed original or unmodulated carrier wave.

Conditions favorable to the production of second order side bands are not necessarily favorable to the production of third order sidel bands or, generally, the conditions favorable to even order modulation are not necessarily favorable to odd order modulation, and vice versa. This is illustrated by Fig. A1 in which A and B are, respectively, the Ec-Ib characteristic. curve and the curve of the second order side band output current of a modulator recently tested. The modulator circuit Was of the general type illustrated in U. S. patent to Van der Bijl, No. 1,350,752, issued August 24, 1920. A tungsten filament carrying a. current of 1.85 amperes was used. kThe plate potential was 220 volts. The important thing is that when the grid is given a polarizing potential of 18 volts, the second order side band becomes substantially Zero. This value of grid potential marks a point of symmetry of the characteristic curve. This condition, which is unusual and dificult to obtain with oxide coated filaments, was made possible by reason of the choice of filament material and the critical values ofthe constants used. The other even order side bands would also be found to be substantially zero if modulation occurred about the same point. Although the odd order side band output is not shown 1n the curve, it is not a'minimum at that point.

The experiment demonstrates that if a characteristic curve having symmetry about means of a filter.

a point is represented by a power series, this series will have no even power terms, that l as the numbers indicating the frequency multiple. This expedient is of value, in its economy of energy, in the avoidance of interferencey due to the presence at the receiver of even order side bands, and on account of its secrecy. Such a system is relatively secret, since it insures that a would-be interceptor cannot readily detect the signal by second order demodulation between the odd order side bands and the appropriate even vmultiple frequency carrier. In multiplex communication systems using side bands of different orders for the respective channels (an eX- ample of which is illustrated in Fig. 3 and which will be described later) the expedient insures a greater separation between message carrying side bands.

Economy and secrecy are promoted in odd order modulation by complete carrier suppression, that is, by suppressing the im pressed carrier as well as its even harmonics. This may be accomplished, for example, by suppressing the even harmonics as above, and suppressing the impressed carrier by Since the odd order side bands are far removed in the frequency range from the impressed carrier, separation can easily be effected by the filter. Suppression of the carrier frequencies may be accomplished in this simple manner without resort to balanced arrangement oftubes. This is of importance in instances where, in the alternative that the usual second order modulatingmethods were attempted to be used with carrier suppression, the carrier frequency would be too high to permit separation from the second order side bands without resort to such balanced arrangements. The method in which modulation is effected by varying the inductance of a coil by correspondingly variably saturating its magnetic core, is especially applicable to suppressed carrier, odd order, modulation. Fig. 3 illustrates one modulating arrangement of this kind and also a system in which it, or any equivalent means, may be used.

Each of the specific frequency values mentioned hereinafter is merely an example of various values which may be employed in a system of the type shown.

In the system of Fig. 3 there are three twoway carrier telephone channels for telephonie communication overa carrier line ML. Of the entire system only one carrier terminal station is shown, slnce the remainder of the system'may bea duplicate of the station and.

connections indicated in Fig. 3 with a slight exception pointed outhereinafter with regard to the source of supply ofV carrier current v at the two stations.

The carrier line ML is associated for transmission purposes with low frequency or telephone lines LJL and L2, through a modulating and demodulating channel 1 and a modulating and demodulating channel 2, respectively. For the purpose of transmission to the carrier line a transmitting and modulating channel TLS, including a telephone transmitter T and a second order magnetic modulator 5,

Iis associated with line ML, through a twoway channel 3; and for the purpose of re- A ceivingfrom the carrier li-ne a receiving and demodulating channel EL3 includinga receiver-demodulator 6 is associated with the carrier line, through the channel 3. A balanced three winding transformer or hybrid coil and an impedance balancingnetwork 8 connect the channels TL3 and RLa to a band filter 31 and in conjugate relation to each other, the filter 31 being interposed between y the hybrid coil and the carrier line. The network 8 balances the impedance facing the h brid coil at the line terminals of the hybrid coil, for frequencies in the neighborhood of the carrier and side band frequencies employed in the modulator 5 and the demodulator 6. The receiver-demodulator employs second order demodulation.

The channel 1 comprises a low pass filter 10 and a band filter 11, a third order magnetic modulator and demodulator 12, a condenser 13 having low impedance for essential lspeech frequencies, and the output side of a carrier filter 14 through which carrier current is supplied to modulator and demodulator 12 by an oscillator 15 in cooperation with a harmonic generator 16 and a harmonic amplifier 17. The oscillator, harmonic generator and 'harmonic amplifier may be of the type usual in carrier current telephone systems. For example, their structure and operation may be as `disclosed in the paper by Colpitts and Blackwell on Carrier. Current Telephony and Telegraphy, Journal of the A. I. E. E., vol. 40, pa e 205. The filters referred to herein maye of appropriate types disclosed in U. S. patent to Campbell, 1,227,113, May 22, 1917.

The modulator 12,-which functions also as a demodulator for channel 1, consists of a toroidal magnetic core wound with a single coil. The function of the modulatoris to distort the currents flowing through the coil sothat third order side bands, one or. both for transmission through filter "11 to line ML, are produced when carrier current is supplied to the modulator through filter 14 and phonictransmission in one direction in line L1 as for telephonie transmission in the opposite direction in line L1. The frequenc of oscillator may be 5000 cycles per secon The harmonic generator 16 delivers harmonics of this frequency, and the frequency delivered by harmonic amplifier 17 and filter 14 may be 10,000 cycles per second. Filter 10 may pass frequencies from 0 to` 3000 cycles. The telephonie signals to and from line L1 may be transmitted over line ML on, say, an upper side band. This will be a third order side band, which will coverthe range 20,000 to 23,000 cycles since the modulation employed for this channel is third order modulation. The filter 11 may pass this latter fr uency ran It will suppress the carrier requencyo 10,000 cycles. The modulator 12, when operating normally for telephonic transmission, will suppress even order side bands and will also suppress even harmonies of the impressed carrier frequency,

includin the first even harmonic.'

What 1s believed to be the mode of operation of modulator 12 will now be explaned byreference to Fig. 2. The curves f and g in this figure are the familiar B-H, or magnetization curves which iexpress the relation between the magnetizing force H and magnetic induction B. Curve f is the magnetization curve for positive variations of H which would be obtained if a magnetic core, as, for example, the toroidal-core in the magnetic modulator, having no initial residual magnetization, were variably magnetized by positive values of current. The curve g would be similarly obtained if negative values of magnetomotive force'were used. Curves f and g, taken together, exhibitlperfect sym.-A

metry about therorigin. The permeabilityv of thecore is measured by the ratio of B and H, that is, by the slope of the chords of the magnetization curve between the origin and the reference points. The curves which ex press the absolute values of permeability corresponding to curves f and g have approximately the shape respectively of curves h and z', which obviously must -be symmetrical about the axis of ordinates. "Since the permeability is a factor/in' the equation which expresses l the inductance of the magnetizing winding, the inducta'ncevaries in accordance with these curves h and 'If the variablesaturation of andthe order of modulation or de,I modu ation employed are the same for telethe toroidal core with variable positive and negative values of impressed potentials is assumed to follow these curves L and z', the characteristic curve between the impressed potentials and current iowing through the coil would be obtained by dividing these positive and negative values of impressed tentials by the ordinates of h and z'. T e resultant characteristic curve would correspond, for exampleto the characteristic curve A of Fig. 1, and would determine the modulating properties of the circuit. The

curve would obviously have symmetry of the same type as that of curve fig, that is, it will have symmetry about a point. Accordingly, under the assumed conditions odd order modulation would be achieved with suppression of the even order side bands l shape of the corresponding permeability curves, they are, as a mattir of fact, of the metry of the loop insures that these permeability curves aresymmetrical. Accord ingly, the operation of the magnetic core modulator, even whenl there is hysteresis, has the desirable features of odd order modulation with suppression of the even order effects.

Of course, the hysteresis loop disclosed would result only if the magnetizing current has not more than one maximum and minimum per cycle, as for example, a sine .same general form as la. and z', and the symwave current. In the actual case the magnetizing current consists of superposed carrier and modulating current, and the resulting wave form accordin ly has numerous irregularities and reversalgs of slope. For this case' the hysteresis loop disclosed illustrates, for example, only the uniformly recurrent loop corresponding to the carrier'current.

The irregularities due to the modulating current would properly be shown as both large and small hysteresis loops having their origins in the loop disclosed.

It is desirable to use a core which saturates at a small value of magnetizing current so that the characteristic cure has pronounced bends at its two extremes, and accordingly so that there are more pronounced high order modulation effects. From another point of View, the use of a saturated core makes possible the production 'the core markedly improves the operation of the device. This metal is an alloy containing two elements of the magnetic group, such group being made up of manganese, iron, cobalt, nickel and copper. In its usual form the metal is an'alloy of nickel and iron. Whatever its specific composition, it is distinguished by its pronounced magnetic properties and especially the remarkably low Value of magnetomotive force required to produce sat-uration. y These magnetic properties depend largely on the special heat treatment which is given to the alloy during its process of manufacture. Reference is made to a paper by Arnold and Elmen in the Journal of the Franklin Institute for May, 1923,and to U. S. patents to G. W. Elmen, Nos. 1,586,883 and 1,586,884, June 1, 1926, for descriptions of this alloy and its properties.

The channelv 2in Fig. 3V comprises a low pass filter 20, a band pass filter 21 and a third order magnetic modulator and demodulator 22 which employs cross magnetization-in its toroidal magnetic core 23 corresponding to the toroidal core of modulator 12 but is otherwise similar to the modulator 12. Core 23 has a winding 24 corresponding to the winding of modulator 12, but only the modulating waves from line L2 and filter 20, or in the case of demodulation a side band passed from the carrier line through filter 21, is applied to this winding, the carrier current being applied to a winding 25 on a magnetic yoke 26 shown with salient poles arranged in close proximity to core 23 to send flux through the latter core at right angles tothe flux due to winding 24. Carrier current having a frequency of, say, 20,000 cycles per second issupplied to modulator 22, for use in modulation or demodulation, from the oscillator 15, harmonic generator 16 and a harmonic amplifier'27, the amplifier 27 selecting and amplifying the 20,000 cycle frequency component of the wave delivered by harmonic generator 16 and applying the amplified wave to winding 25.

The function of modulator 22 is to generate third order side bands, one for transmission through filter 21 to line ML, when carrier current is supplied to the winding 25 from amplifier 27, and telephonic current is supplied to the winding 24 from line L1 through filter 20, and to generate speech frequency waves when carrier c-urrent of 20,000 cycles per second is supplied to the modulator from amphfier 27 and a -third order speech side f band of a carrier current of 20,000 cyclesis supplied to the modulator, from the carrier line, through filter 21. In channel 2 the carrier frequency and the order of modulation or demodulation employed are the same for telephonic transmission in one direction over line L2 as fortelephonic transmission in the opposite direction in line L2. Filter 20 may pass frequencies from 0 to 3,000 cycles. The telephonic signals to and from line L2 may be transmitted over line ML on, say, an upper side band. This will be a third. order side 43,000 cycles since the modulation employed for this channel is third ordermodulation. The filter 21 may pass the latter frequency range'. It will suppress the carrier frequency of 20,000 cycles. The modulator 22 will suppress even order side bands and will also suppress even harmonics of the impressed carrier frequency, including the first even harmonic.

The magnetic coil modulator 12 illustrates what may appropriately be denoted longitudinal magnetization, since the fiuxes resulting from the two magnetomotive forces follow parallel, that is, the same paths. It has been found that a similar effect is produced by what may be called cross-magnetization in which the windings are so arranged as for* example in the manner illustrated in modulator 22, that the two fluxes tendto be normal to each other. Thethree circuits for the modulating, carrier and side band currents may be otherwise related as in the case of the modulator 12, or in any suitable manner. The use of cross magnetization has the advantage that loo the permeability, as well as physical structures for accomplishing cross magnetization are described in an article by Goldschmidt in the Elektrotechnische Zeitschrift for March 3, 1910, page 218.

Carrier current of say 10,000 cycles per -second is supplied to modulator 5 and demodulator 6 from elements 15 and 16 and a harmonic amplifier 37, through carrier filter 34, channel 3'and hybrid coil 7. A low pass filter 30', having a pass range of say 0 to 3,000 cycles is interposed between transmitter T and modulator 5. Telephonie signals to demodulator 6 and from modulator 5 may be transmitted over line ML on, say, an upper side band of the 10,000 cycle carrier. This Will be a second order side band, with a range of 10,000 to 13,000 cycles. Filter 31 will pass this range.

The station at the distant end (not shown) of lineML may be a duplicate' of the station 10 shown, except that the oscillations for. the

harmonic generator at the distant station,

corresponding to harmonic generator 16, are

supplied from the oscillator located at the station shown, through a base frequency am- 15 plifier- 57, a base frequency filter 51 and the carrier line ML. The base frequency amplifier selects a wave of 5,000 cycles from element 16, amplifes the wave and transmits it through the 5,000 cycle filter 51.

A relay 60 is selectively responsive to ringirlig current, of say 16 cycles, from line L1, to e ectively open the circuit .of a relay 61 and thereby cause a' battery 62 or other source of `direct current to send current through modulator 12, a choke coil 63 which has high impedance for speech and carrier frequencies, and left hand contact of relay 61. This current subjects the core of the modulator to a unidirectional magnetomotive force for the duration of the ringing current. The resulting biasing flux or unidirectional fiux in the modulator core whichis also subjected to the magnetomotive force due to the carrier current, causes the modulator to shift its operatother than that about which the curve is symmetrical, and consequently the modulator delivers a pronounced rst even harmonic of the carrler frequency' 10,000. This- 20,000 cycle wave asses through the band filter 11, and over tlie carrier line, and through the corresponding band filter at the distant carrier terminal, and there operates, in a manner -which will now be explained by reference to the station shown, to sen'd a ringing current over the low frequency telephone lline terminating at the station.

When a wave of 20,000cycles is received from the carrier4 line byfilter' 11, the wave is transmitted to a magnetizing coil or winding 65 of a relay-demodulator 66, the coil 65 :being connected in arallelwith condenser 13 and the coil of mo ulator 12 by a circuit which extends through the normally closed right hand contact of relay 61 and through two parallel branch circuits 67 and 68. The' branch 67 contains a phase changer 70 between narrow band filters 71 and 72 which pass the frequency 20,000 cycles.v The branch 68 contains a narrow band filter 73 which passes `the frequency 10,000 cycles received from harmonic amplifier 17 through carrier filter 14. The relay-demodulator is a relay -comprising a magnetic core 75, energized by the winding 65, and an armature 76.' In the ing point on its characteristic curve to a point closed core 75 demodulation of third order takes place between' the 10,000 and 20,000 cycle waves applied to winding 65. If the two waves be 1n proper phase relation, one of the demodulation products is a term. roportional to cos2 pt cos 2 pt, where p is 2frt1mes the frequency 10,000. Therefore, since f cos2 7:15:1/24-1/2 cos 2 pt, it followsthat there is a undirectional flux produced in the relay core, though there is nodirectcurrent in the relay winding. This unidirectional flux causes the armature 76 to close its contact and` thereby energize a relay 77 which connects a source 78 of 16 cycle ringing current to line L1. By

having the circuit of Winding 65 pass through the front contact of relay 61, 1t is insured that this circuit will be open whenevermodulator 12 is generating a 20,000 cycle wave due to application of the voltage of battery 62 to the modulator 12. The armature of relay 66 should be unresponsive to alternating fiux, and particularly to high frequency flux. The mention above of the proper phasing, which may be accomplished y'by varyin the setting of the phase changer until a suicient force for operating the relay armature is obtained,

has reference to the fact that at a certain thereon windings 81 and 82 for energization by side band'current delivered from filter 31 and lwindings 83 and 84 faorl energization by a direct current source 85. The receiverdemodulator also comprises vmagnetic polar projections 86 and 87 protruding from the core 80 at points 88 and 8 9 respectively and a magnetic sound reproducing diaphragm mounted forvibration by the flux from the polar (projections 86 and 87. The windings 81 an 82 have the same number of turns and are at opposite sides of the point 88 and are connected in series aiding relation, with respect to the closed flux path 80. The winding 83 has the same number of turns as winding 84, and is onk the opposite side of point 89, and these two-windings are in series o posingl relation, with `res ect to the ux pat 80. The .core 80 is symmetrical about a plane passing through points 88'and 89 at right angles to the plane of the closed magnetic path, and such a plane may be re rded as dividing the core or path into two alves, the reluctances of which are equal. Point 88 is at the same magnetic po' tential as point 89, as re ards carrierand side band magnetomotive orce produced by windings 81 and 82, and therefore no iuxes of the carrier and side band frequencies How through the projections 86 and 87. Winding 83 sends a unidirectional ux through the upper half of core 80, in one direction around the core 80, and -winding 84 sends an equal unidirectional flux through the lower half of core 80, in the opposite direction around the core. These two uxes rcturn in the same direction through the path consisting ofthe projections 86 and 87, the diaphragm 90 and the air-gap between the diaphragm and the free poles of the projections.

The second 'order intermodulation of the impressed carrier and side band iiuxes produces in each half of the closed magnetic path or core 80 a signal frequency flux which in magnitude and sign is dependent upon the algebraic product of4 the magnitudes of the carrier frequency iux, the impressed side band flux and the direct current flux. Since the direct current fluxes are of opposite sign or direction inthe two halves of the core, the two algebraic products or signal fluxes of equal magnitude have opposite directions in the core. Therefore they iow in the same direction through the path consisting of the projections 86 and 87, the diaphragm 90 and the air gap at the'diaphragm, and conseuently cooperate with the direct current ux throughthe air gap to exert upon the diaphragm a vibromotive-force substantially proportional to the product of the direct current flux and the sign-alflux resulting from demodulation. p

In Vthe operation of the system of which Fig. 3 forms one of two similar carrier terminal stations the channels 1, 2 and 3 provide for three two-way transmissions of telephone messages simultaneously over the carrier line, the transmissions between the carrier line and the lines-L1 and L2, respectively, employing third order modulation, and the transmission from the telephone transmitter T to the carrier line employing second order demodulation as does also theA transmission from 'the'carrier line to the telephone receiver 6p In each of the channels 1, 2 and 3 the carrier frequency is the same for one direction of transmission as for the opposite direction. For transmitting ringing signals from line L1 to the carrier line, ringing responsive relay 60 causes a biasing magnetomotive force to be applied to the modulator 12, so that the modulator sends a wave of double the impressed carrier frequency to the carrier line. For transmitting ringing signals from the carrier line to line L1 the relay-demodulator 66 respends to the double carrier frequency wave representing the ringing signal, and vcauses a 16 cycle generator to be connected to line .wave with suppression o the carrier wave,

and transmitting a'second signal, other than that employing said vside frequency,'by re-I introduction of said carrier wave.

2. The method which comprises generating a iux by intermodulating magnetomotive forces, and controlling a flux responsive mehanical device in response to the generated 3. The method which comprises producing a flux having a unidirectional component by application of alternating magnetomotivev forces to a magnetic path, and applying said flux to a device selectively responsive to said unidirectional component of said flux. A

4. A signaling system comprising an odd order modulator,l means for impressing a carrier wave and a modulating wave on said modulator, means for causing said modulator to generate an even harmonic frequency of said impressed carrier wave, a carrier line for transmitting a side frequency wave generated by said modulator in response to said carrier and modulating waves, an electro-` magnetic relay, connections for transmitting said side frequency wave from said modula- ,tor to said carrier line and for transmitting waves of the frequency of said harmonic from said carrier line to -said relay, and means for supplying to said relay waves of the frequency of said carrier wave.

5. A system in accordance with claim. 4, comprising a telephone line in said means for impressing said carrier and modulating waves on said modulator, and comprising a relay responsive to ringing `current from said telephone line for controlling said connections, a source of ringing current and means controlled by said electromagnetic relay for connecting said source to said telephone line.

6. A communication system comprising a plurality of two-way multiplexed carrier telephone channels, a carrier line for said, channels, third order magnetic modulating and demodulating means for one of said channels at each terminal of said carrier line, second order magnetic modulating and demodulating means for a second of said channels at each terminal of said carrier line, p

vwave with suppression o for magnetically biasing one of said third order modulating means to cause it to generate the first even harmonic of the carrier frequency impressed thereon, a ringin current generator at .the distant terminal o the carrier line, and means at said distant terminal responsive to said harmonic to control said ringing current generator.

7. A signaling system comprising means for generating and transmitting a side frequency of a carrier wave with suppression of the carrier wave, and means for reintroducing said carrier wave to convey a signal other than that identified by said side fre-- side frequency.

10. A si naling system comprising an odd order moulator, means for impressing a carrier wave and a modulating wave on said modulator, means for transmitting a side frequency generated by said modulator, means for blasing'said modulator causing it to enerate a harmonic of said carrier wave, and means for utilizing said harmonic wave for transmittin a signal other than that employin said si e frequency.

11. he method which comprises generating an alternating flux by distorting superimposed magnetomotive forces and controlling a iux responsive sound producing device in response to the generated flux.

12. A combined telephone receiver and demodulator comprising means responsive to carrier and modulated currents for producin tive orces, means for demodulating said magnetomotive forces to produce a demodulated signal flux and vibratory sound proucing means operable in response to said 13. The method of producing a signaling wave which comprises subjecting a magnetic path to magnetomotive forces which are substantially at right angles to each other throughout said path and the magnitude of one o which is a function of the magnitude of a signaling wave and the magnitude of the other of which is a function of the magnitude of another wave whereby a magnetic flux is generated and controlling a flux responsive mechanical device in response to the generated flux.

14. The method which comprises jointly impressing on a magnetic path a carrier wave carrier and modulated ma netomo`4l of magnetoniotive force and a si aling wave of magnetomotive force at rig t angles to said carrier wave and utilizin the resultant iux comprising side bands o' higher orderto control a flux responsive mechanical device.

15. The combination with lan electromag- In witness whereof, I hereunto subscribe l my name this 29th day of October, A. D. 1926.

EUGENE PETERSON.

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